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Fix SlotNotCoveredError when cluster is resharding #2989

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Fix SlotNotCoveredError when cluster is resharding #2989

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c0e6e2b
Fix SlotNotCoveredError when cluster is resharding
dinispeixoto Oct 6, 2023
196f439
Merge branch 'master' into fix-slots-not-covered-error-when-cluster-i…
dinispeixoto Oct 13, 2023
f177f04
Merge branch 'master' into fix-slots-not-covered-error-when-cluster-i…
dinispeixoto Oct 27, 2023
d31e0f3
Fix logic to check if index is in slots cache list
dinispeixoto Oct 27, 2023
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6 changes: 5 additions & 1 deletion redis/asyncio/cluster.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1195,7 +1195,11 @@ def get_node_from_slot(
node_idx = self.read_load_balancer.get_server_index(
primary_name, len(self.slots_cache[slot])
)
return self.slots_cache[slot][node_idx]

# we use the node returned by RR in the load balancer
# if it's part of the slots cache, otherwise we use primary
node = node_idx if node_idx < len(self.slots_cache[slot]) else 0
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@johan-seesaw johan-seesaw Nov 15, 2023 •
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The only case I can see this happening, is the LoadBalancer object having a pre-existing history with a list_size of lets say 3 nodes for the given primary. If an event happens where the list is no longer 3 (lets say new list is size 1), and we enter LoadBalancer with the existing dictionary having a value of 2 for this primary, then we will return 2 from the get_server_index method, but it will perform a % 1 operation before storing the "next value" in the dictionary.

Perhaps a simpler rewrite would be to store the last-used value, not the next-used value in the LoadBalancer class, then there would only be one modulo operation, and we would always be performing it with the current list size.

    def get_server_index(self, primary: str, list_size: int) -> int:
        # default to -1 if not found, so after incrementing it will be 0
        server_index = (self.primary_to_idx.get(primary, -1) + 1) % list_size
        self.primary_to_idx[primary] = server_index
        return server_index

return self.slots_cache[slot][node]
return self.slots_cache[slot][0]
except (IndexError, TypeError):
raise SlotNotCoveredError(
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